1. Field of the Invention
The present invention relates to a mobile communication system and, more particularly, to a mobile communication system allowing a mobile station to acquire a pilot channel rapidly when moved from an area using an analog system to an adjoining area using a CDMA (Code Division Multiple Access) system.
2. Description of the Background Art
It has been customary with a cellular mobile phone system to use an AMPS (Advanced Mobile Phone System) or similar analog cellular system. Today, however, digital cellular systems are under study and development because the analog cellular system cannot cope with the overwhelming number of subscribers. Among the digital cellular systems heretofore proposed, a CDMA cellular system is attracting increasing attention because it is capable of spreading the frequency bandwidth of signals to be transmitted up to about 1.25 MHz by a spectrum spread technology. With the CDMA system therefore, it is possible to implement a system having a huge capacity on the basis of delicate control over transmission power, various kinds of diversity effects, soft handoff, and so forth.
In the CDMA cellular system, signals of the same frequency to be sent from a base station to mobile stations are each spread by a particular spread code. The resulting signals with a spread spectrum are superposed and sent to the mobile stations. Each mobile station despreads a received signal spread by a desired spread code by use of a local spread code synchronous with the desired spread code. As a result, interference waves are despread and turn out noise, while only a desired signal is demodulated in a narrow bandwidth.
In practice, the transition from the analog cellular system to the CDMA cellular system will be originated in areas accommodating a great number of subscribers. Presumably therefore, an analog system using the analog cellular system and a CDMA system using the CDMA cellular system will coexist for the time being, and may even adjoin or overlay each other. A subscriber intending to receive services from both the analog and CDMA systems must use a mobile station capable of communicating with the two systems. When the mobile station with such a capability moves from an analog system area to a CDMA system area, the user of the mobile station is expected to end conversation based on the analog system, and then switch the mobile station from an analog mode to a CDMA mode. After the mobile station has become ready to receive an incoming call based on the CDMA system, the user is allowed to again originate a call and resume the interrupted conversation.
The prerequisite with the CDMA system is that the local spread code generated in the mobile station be coincident in timing with the spread code generated in the base station (synchronization), so that a desired received signal can be demodulated. While various methods have been proposed for the synchronization in the past, serial search acquisition is a practical method available at the present stage of development. The serial search acquisition is such that the mobile station generates a spread code at a suitable timing first, and then sequentially shifts the timing until it coincides with the timing of the spread code being received from the base station.
However, the problem with the serial search acquisition scheme is that a substantial period of time, even one period of the spread code in the worst case, is necessary for the synchronization to be set up. Particularly, in the CDMA system whose spread code has a long period, a critically long period of time is necessary for the synchronization to be set up. For example, the evaluation report of actual field tests conducted with a CDMA cellular system prescribed by the North American Standard IS-95 teaches that the synchronizing time was as long as 10 seconds to 15 seconds.